Cold cathode ionization vacuum gauges are well known. Three commonly known cold cathode ionization vacuum gauges include normal (noninverted) magnetron type gauges, inverted magnetron type gauges, and Philips (or Penning) gauges. All of these types of gauges have a pair of electrodes (i.e., an anode and a cathode) in an evacuated non-magnetic envelope which is connected to the vacuum to be measured. A high DC voltage potential difference is applied between the anode electrode and the cathode electrode to cause a discharge current to flow therebetween. A magnetic field is applied along the axis of the electrodes in order to help maintain the discharge current at an equilibrium value which is a function of pressure.
Accordingly, a cold cathode ionization vacuum gauge (CCIVG) provides an indirect measurement of vacuum system total pressure by first ionizing gas molecules and atoms inside its vacuum gauge envelope and then measuring the resulting ion current. The measured ion current is directly related to the gas density and gas total pressure inside the gauge envelope, i.e., as the pressure inside the vacuum system decreases, the measured ion current decreases. Gas specific calibration curves provide the ability to calculate total pressures based on ion current measurements.
A significant difference between a CCIVG and a hot cathode ionization vacuum gauge (HCIVG) is the lack of a hot filament to establish an ion current in a CCIVG. The lack of a hot filament simplifies the construction and operation of the CCIVG and improves its reliability, as there is no risk of filament burn-out by sudden or accidental exposure of the gauge to a high pressure. The lack of a hot filament, on the other hand, complicates gauge monitoring as there is no independent electron current to be measured and controlled, unlike in a HCIVG where the electron emission current is monitored and used to assure the validity of the ion current measurements. In other words, as the CCIVG starts to lose sensitivity, both the electron and ion currents decrease over time; however, since the user does not have direct access to electron current (in contrast to a hot cathode gauge), there is no way to know whether a drop in ion current is due to a reduction in electron current in the discharge or a reduction in process pressure.
In cold cathode ionization vacuum gauges of the inverted magnetron type, it is possible for a small leakage current to flow directly from the anode to the cathode via the internal surfaces of the gauge, and it is known that the presence of a so-called “guard ring” can collect this leakage current and thereby prevent it from reaching the cathode electrode and being detected by the gauge itself. To perform this function, the guard ring is electrically isolated from the cathode electrode and normally held at a small positive voltage potential difference relative to the cathode electrode.
Another aspect of cold cathode ionization vacuum gauges is that, as the pressure decreases, the gauge can take longer and longer times to start the discharge that is used to provide the ion current that is used to measure pressure. Many designs have been used to seed electrons into the discharge volume to trigger the avalanche process that is responsible for building up the discharge.
Nevertheless, there continues to be a need for improved cold cathode ionization vacuum gauges that minimize or eliminate the problems described above.